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Why isn't the U-2 used for commercial purposes?
There are some companies that use decommissioned military aircraft for commercial high-altitude flights (for, say, tourists). Examples would be the MiG-31 and the MiG-29:

Why isn't the Lockheed U-2 used for such flights? Here is a two-seat version of the U-2:

( Image source )
Is the U-2 still secret? Or is it expensive to build?

it's nearly entirely titanium... also what commercial purposes would you envision?

It would cost more than likely anyone is likely to pay.

@ratchetfreak: Titanium? Aren't you confusing it with that other Skunk Works reconnaissance plane?

Excellent! Thank your for sourcing your image.

It is expensive to operate, and there is no economic value in what it does.
Apart from reconnaissance and espionage, the U-2 is used for high-altitude atmospheric research, but since it is operated by NASA and man-carrying, it is subject to an immense bureaucracy. When you want to fly your own scientific instrument on it, you need to go through years of certification procedures. The instruments which are rated to fly on the U-2 are ancient and much less precise than what can be built today. Therefore, most high altitude research is done by balloons, where the bureaucracy is much less suffocating.

I meant something like taking tourists to the edge of space.

@user3624251: Except for the view and the custom-fit suit, there is not much spaceflight feeling in operating the U-2. No weightlessness, no fiery reentry, but much boredom.

@user3624251 It was used for a "tourist" at least once - Adam Savage of Mythbusters got to take a joy ride on a U2.

@T.J.L. I'm aware of two instances of people riding shotgun in a U-2 for television: James May  for a BBC special and Adam Savage, for an episode of Mythbusters.

@ThomasOwens James May, too, huh? I'm not surprised. I'll have to try to dig that up.

I don't necessarily agree with "instruments which are rated to fly on the U-2 are ancient and much less precise than what can be built today". I don't know what payloads NASA flies on their U-2, but the USAF payloads are quite modern. In fact, their payloads are still being upgraded on a regular basis. Some are more incremental, but they are by no means ancient. From what I've seen, it's much easier to get new payloads and technology onto a government or military aircraft than a commercial aircraft, in terms of certification. Just from what I see on a day-to-day basis.

Here it is, James May in U-2: https://www.youtube.com/watch?v=jIJRoj2qwsc

@ThomasOwens The first unmanned aircraft I worked on in my career was on purpose designed to put modern instruments into the stratosphere inside the southern polar vortex. Those instruments would not be touched by NASA, but they proposed man-rated, ancient, heavy instruments of lower precision. The U-2 would had operated out of Punto Arenas, making each flight expensive and limiting the top altitude. Think about it: The researchers would rather build a new aircraft than accepting the rotten deal NASA was offering.

"Suffocating"... really? You had to use that word when talking about high-altitude work? :)

Because the owners of the U-2s (the USAF and NASA) don't make them available for commercial use. Presumably, the U-2s on display in Norway and the UK and the US aren't operable and were supplied on the condition that they wouldn't be operated.


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A320neo family IATA codes
I'm about to ask a question that's already been asked but that wasn't answered. I was torn between opening a thread thats many months old or posting a new thread. I apologize if I picked the wrong option.
Previous thread: What is the IATA code for the A320neo family?
What are the IATA codes for the A320neo family?
According Lufthansa, A320neo (ICAO: A20N) is 32V. But that's all that I know.

Welcome to aviation.SE! Please don't ask a question that has already been asked. If there are no good answers already, then the best thing to do is to upvote the original question to attract more attention. You might like to take the [tour] if you aren't familiar with his this site works.

@Pondlife Vote up is 15 rep, and set bounties is 75 rep (and certainly wouldn't be meaningful below 51 rep, anyway, since the least you can bounty off is 50...). I've found that a constructive edit to an old post (a question or any of its answers, if any) can often draw renewed attention to it as well, but one has to be careful in doing so to not change intent.

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What is the reason for this engine oil service procedure on a Bombardier CRJ?
Someone told me that servicing the engine oil on a Bombardier CRJ900 aircraft with the APU on could lead to the particular engine being overserviced. He then advised that the engine-oil service be done within 20 minutes of aircraft shutdown.
What's the reason for this?

You should edit the title to transform it in a question (this is a Q&A website). It will help browse questions about oil services without opening each question

Pure speculation, but could draining the oil from an engine leave the APU with insufficient oil? I've no idea what 'over serviced' would mean.

Please clarify: do you wish to service the APU oil with the APU running, or service the engine oil while the APU is running?

Engine Oil Service with the APU running.

Is there any head pressure applied to the oil reservoirs using bleed air? Then if APU is running, it is providing this pressure.

@RobinBennett, the usual meaning of “over servicing” in this context is overfilling, i.e. adding more oil than you should.

When the engine is running, it sucks up the oil from the reservoir and puts it in all the galleries, ie the oil's all in the right places to lubricate the engine.
When you shut the engine down, the oil starts returning to the reservoir by gravity. So there's a gentle drip-feed back into the reservoir, which means that the apparent oil quantity measured at the reservoir is increasing.
So the problem is WHEN should you measure the oil quantity - when just before the oil starts returning to the reservoir, or after that process has completed? Well, it can take a while, so the general convention is to read the oil quantity, and to add oil as necessary, before the oil has gone back to the reservoir.
(Don't know why the APU has anything to do with this.)

That shouldn't be a matter of convention, but rather be clearly indicated in the user or maintenance manual.

I understood the OP has asked about the service while the APU is running. So it's like a third engine which also would affect (decrease) the oil level in the reservoir.

@PerlDuck, I am somewhat confident each engine has a separate reservoir and so does the APU. Because if one leaks out, you don't want it to disable all of them.

The Aircraft Flight Manual or Flight Crew Operating Manual will specify the timeframe during which engine oil may be checked after engine shutdown. For the CF34-3B engine on a Challenger 605, checking must be done between 15 minutes and 2 hours following shutdown.
If done outside of this window, the engine must be dry-motored (cranked without introducing fuel) for at least 30 seconds.
Otherwise the oil level may (incorrectly) show too-low (the oil tank is not at the bottom of the engine), which could lead to overfilling. Overfilling is said to be worse for jet engines than operating them with slightly too-low oil levels.

I don't know the specifics for that plane, but in general you do oil changes with the oil warm so that it is less viscous, so there's a minimum amount of dirty, old oil left inside the engine (and so you don't have to wait hours for it to drain).
However it's generally a bad idea to do it while the engine is still at operating temperature, because then the oil will be hot enough to cause serious burns as you remove the drain plug.

Which explains the second half, but not the first.

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How did lateral and longitudinal separation minima evolve?
What is the evolution in time of lateral and longitudinal separation minima?

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Why is the nosewheel of the MiG-29 placed in front of the engine intakes?
Why do MiG-29 have a front-wheel of landing gear placed in front of an intake? would not it throw water or stones into the intake as mentioned in Raymer?
I am designing an aircraft based on the reference aircraft MiG-29.
I have used an RD33 twin-engines with twin intakes placed in chin position beneath the fuselage. Which design configurations should be kept in mind?
The intake I have selected is a ramp intake.

apparently not!

Would be hard to put the nosewheel behind the engine intakes, yes? Unless they were moved forward a lot. I suppose the nose strut hinge point could be moved back to swing the strut forward if there were enough space under the pilot for the wheel well.

Highly related answer in which you can see intake protections.

A good answer may mention how far from main gear the nose gear can/should be.

To confirm awnsers: https://youtu.be/FjuzYaqXpws

Because if they were placed much further back, the aircraft would tip onto its nose if the brakes were applied?

The MiG-29 series of aircraft are designed to operate from poor quality airfields, and for this reason, they feature separate air intakes for use while moving on the ground.
See this article, including pictures of the aircraft:
https://www.airforce-technology.com/projects/mig29/
Notice the additional louvre-shaped intakes on top of the aircraft's leading edge extensions.
Quote from the article:

The MiG-29 is the world’s first aircraft fitted with dual-mode air
  intakes. During flight, the open air intakes feed air to the engines.
While moving on the ground, the air intakes are closed and air is fed
  through the louvres on the upper surface of the wing root to prevent
  ingestion of foreign objects from the runway. This is particularly
  important when operating from poorly prepared airfields.

In general, aircraft designers will have to consider factors such as the aircraft's weight distribution and center of gravity when choosing a position for the landing gear.
If you compare with other fighter aircraft of similar layout and configuration, such as the F-15 or the F-18, you will notice that most aircraft have the landing gear approximately in line or in front of the air intakes, mostly because either changing the landing gear position or the length of the air intakes only to move the landing gear behind the air intakes would likely create other problems, such as undesirable taxi characteristics, unstable takeoff/landing roll or undesirable aerodynamics in flight.
The configuration where the landing gear is approximately in line or in front of the air intakes, on the other hand, has been used successfully on many existing types, so it can be assumed that there are no major problems that would be solved by moving the landing gear further back.
As it can be seen on the MiG-29, there are also other ways to avoid specifically dirt or foreign objects ending up in the air intakes during takeoff and landing.


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Why is there no video of the drone propellor strike by Russia
I've seen lots of videos of the Russian aircraft flying near the US drone, but I've yet to see a video of the reported collision with the drone's propellor.  Is there no video of the strike?  If not, is there a reason why the video would stop working at the time of the strike?  Some of the videos appear to show a strange color pattern and stop working when the Russian aircraft gets close.  Would that be some kind of EM capability used by Russian aircraft?

I believe UAVs like this, send their signal from antennas on the top of the aircraft, up to a satellite. A large metal object (Su-27) would momentarily disrupt that. Possibly they also have some onboard video storage, but that has not yet been recovered.

I am hypothesising rather hugely about this, as the details are not in the public domain, but the UAV in question (the MQ-9) is controlled via satcom through a bulge on its back, as shown in this lovely picture from Wikipedia:

The satellite requires stable alignment and accurate tracking -- it is highly directional and you can approximate its angular resolution very roughly by assuming that the dish is about a foot wide and X-band microwaves are being used for communication. This gives, as an order of magnitude estimate of the required directional accuracy, $1.22\lambda/D\sim 1.22\times 3\,\mathrm{cm}/30\,\mathrm{cm}\sim\mathcal{O}(5°)$. This is really quite a precise alignment in absolute terms – look at a protractor and imagine e.g. holding your arm out in a moving car pointing at a dot and not deviating from it by that much.
The slightest jar – such as being hit – will therefore knock the communication offline while the satellite's tracking is able to re-establish itself with the requisite location. Note that 5° is actually quite a loose alignment target for satellites – anyone who has put up a TV satellite dish in their garden will know that for commercial TV stations a tolerance of ~0.1° is needed. High altitude, higher-bandwidth military systems can get by using a higher frequency part of the spectrum – I understand that they use X-band (assumed above) or Ka-band mm-wave for these sorts of communication requirements. Either way, the dish requires precise alignment and control over it (hence all the motors and electronics in the above).
After the knock, I suspect what is seen in the interim is a test signal relayed to illustrate that everything (i.e. the encryption protocol) works from the ground to the satellite. It's also worth mentioning that there is a reported 1.2 second delay between either the drone transmitting and the operator receiving or vice versa, so it wouldn't have been possible for the operator to take evasive action easily or safely. The dead time in the video of the collision is about ten seconds, indicative of a longer steering process of the dish.
I also feel compelled to note that there is clear evidence of damage to the propeller in the "after" portion of that video, however: one of the tips is bent and missing:

If there is video of the strike proper, it's at the bottom of the Black Sea.
There was no ability to transmit video of the strike because the first effect of the collision (or even pre-collision wake effects) was to knock the satellite dish out of alignment, as Landak says.
So the only recording would be of any on-board recording done on the drone.   However a few factors make it unlikely to ever see such data:
First, the US was probably able to regain flight control of it, long enough to do a "wipe" procedure deleting all sensitive software and data. It is likely this video recording was among the data wiped.
Second, the evidence from the video has every aviation expert recognizing an accident from a careless pilot* - disturbingly similar to the Texas B-17 loss of 2022(

) where a fighter did exactly the same thing - put themselves belly-up to the other aircraft, making it impossible to see to avoid them. That's why the US government is treating it like a jackass stunt gone wrong, and not an act of war. The facts support that.   But this informs your question of why we're not likely to see any gun camera from the Russian side: there are only 3 possible cases, and all 3 give the Russians strong incentive to share or not.

it conforms sheer incompetence - out of pride they'd never share it.
it and CVR shows intent to destroy - that would be even worse!
it shows actual innocence, and American incompetence or malice e.g. elevator movements on the drone to pitch it up into the Russian fighter's path - in which case they would be eager to share it!  That hasn't happened, so we can bet that is not the case.

"There is absolutely no tactical advantage to risk losing an Su-27 over an MQ-9 drone by intentionally hitting it" - Juan Browne
So yeah, risking loss of a Su-27 to get an MQ-9 would be a lousy trade.  Further, if that had been Russia's intent, they have an excellent recovery ship - the Kommuna, the oldest naval ship not in a museum - and it's in the Black sea.  If the Kommuna and other vessels had been pre-positioned for salvage operations, this would be a different kettle of fish.

"an accident from a careless pilot" -- I don't want to get too far into political weeds, but "accident" seems too generous a term; the collision itself may have been accidental but the close pass and fuel dump was clearly malicious.

"That's why the US government is treating it like a jackass stunt gone wrong, and not an act of war." It seems to me that those aren't mutually exclusive, at the extremes. At some point, it becomes a case of "I'm just going to walk towards you swinging my fist, and if you get hit, it's your own fault."

What does "hull-down" mean?

One reason for the US government to treat it like a jackass stunt gone wrong, is that if they treat it as an act of war, they'd be under pressure to retaliate, and they would prefer to avoid escalation.

@Acccumulation Sorry, I removed that and used verbiage that conforms to the linked video.

@Russell Well I suppose one can make anything of anything, like the way they made WWI out of Archduke Ferdinand's assassination but not WWIII out of JFK's. But I'm sure the US has other stuff they're not sharing, like the radio chatter between SU-27s and planes to base, or more precious intelligence that informs the question.  Also, the US was already very badly burned the last time they twisted intelligence and 'wagged the dog' -- with the WMDs of Iraq.  The US has to be a reliable teller of truth or the rest of the world will roll their eyes and go "there you go again".

Because an SU-27 does not have a forward facing video camera as standard equipment as opposed to the turret mounted video camera on an MQ-9 UAV.
The SU-27 flight crew either were instructed not to carry, or opted not to carry, commercial video equipment such as a GoPro camera, etc. That could have filmed the mid air collision.
If such a video was shot, Russia may quietly be withholding it from release in light of the fact that this has created an international incident where it is culpable.
But the MQ-9 did, in fact, capture footage of the MAC with the Russian Flanker and it clear as day shows damage to one of the propeller blades on the drone.

Actually, I'm not sure anyone has pointed this out yet, but it can be seen that two propeller blades are damaged, not one.  Look near the very end of the video.  The blade directly opposite the obviously damaged one, can be seen to be knocked to a different (closer to feathered) pitch angle, and slightly bent as well.  Only visible through one single pass of the blade through the camera view, just before video ends.

(I don't know whether there's any possibility that the blade could be actually rotated as a single unit to a different pitch angle, but it sure looks like it-- grounds for a new ASE question?-- anyway, if not, it's somehow been bent in such a way as to create the same effect.)

Are you saying you could see the MAC in this video?  I can see the jet pass nearby, but when it gets close the video is garbled, and sometime later it comes back on with damaged blades evident.  Just wondering why the video stopped working as the jet approached from the aft, then started working again after the jet flew over.

The MAC occurs as the video temporarily goes to the test pattern (Flanker really rattled its cage there).


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Is quadcopter design scalable?
I've been recently introduced to the world of mini aircraft, specifically, quadcopters - building them, flying them, calibrating them (apparently, it's such an important point that it deserves its own place on the list :) .
I'm about to finish building my own quadcopter (as soon as I can get a power distribution board - hard to find, it seems), and my hands are itching to try and fly it as soon as possible. I've learned a lot about how planes / helicopters / quads fly while in the process of building one. But maybe I haven't not learned enough about it, and was hoping you can help me with one question I have.
I think I've learned enough about the types of propellers used in these vehicles, power requirements, controllers, calibration, and whatnot. While searching about these things on the net, and asking a few friends about their experiences, a thought came into my mind:
Wouldn't be nice if I could, basically, scale all the components of this small, 330mm vehicle I'm building, to make something bigger?
I mean, bigger as in big enough to carry one, maybe 2 people around ?
Can I really say "hey, let's build a frame X times larger, with stronger materials, lets get 4 motors X times bigger, let's use 4 X-sized variable pitch propellers" and so on and so on, and actually be able to build a quadcopter able to carry people in it?
(Yes, yes, I know I'm basically saying "let's build an actual size helicopter, but with 4 propellers instead of one".)
Do you think this is a feasible approach to build such a thing? Is it even remotely possible? Have you had any experience doing such a thing? Any comments will be highly appreciated.

Well, there are a lot of concerns with doing this, though it still may be technically possible.  You should check out the other question cpast linked for a further discussion.  And, if you have more specific questions regarding quad copters, please feel free to post them here :).

Ducted fan designs like quadcopters are actually somewhat simpler in some ways to design and build than single-rotor helicopters, because the nature of the design means that the rotors don't need the complex mechanical linkages of a helicopter's main rotor; the four rotors can control pitch and bank simply by varying RPM, and so each one can be a fixed-pitch propeller. The design becomes more stable with a variable-pitch rotor design as the possibility of varying thrust with a constant rotor speed reduces unwanted roll moments from gyroscopic forces, but that throws the simplicity argument out the window (and helicopters have potential roll problems during throttle-up as well, some of which had to be solved before the thing could even take off).
Yaw is a little trickier, and one potential limiting factor to scaling up the design. The problem is that with all four rotors in a fixed vertical orientation, which is the simplest mechanically, there's nothing to produce lateral thrust to turn the aircraft around the Z-axis (straight up from the ground through the 'copter in level flight). Options include:

One laterally mounted fan. Simple to design, but you're adding one more fan to a design with a lot of high-speed spinning parts already, and the fan would either have to stop and start in opposite directions very often (easy with a little DC motor and plastic fan on a model, much more difficult when scaled up), or the blade pitch would have to be variable, to allow variable thrust to either side while the rotor turns in one direction) which increases complexity again.
Two lateral fans, one facing each direction; by varying their power (they'll probably both be spinning at a low idle while in flight) you provide yaw. Advantage is mechanical simplicity of each fan, disadvantage is having not four, not five but at least six fans, reducing "mean time between failures" of at least one of them increasing required maintenance that much more to ensure it doesn't happen in flight.
Arranging the main four rotors such that each rotor rotates the same way as the rotor in the diagonally opposite corner, and opposite the other two. By reducing the power of fans rotating in one direction and increasing the power of the other, you can induce yaw as the craft will yaw opposite the direction of the faster rotors due to the torque imbalance, without causing the aircraft to also pitch or roll as the total forces in those planes stays constant. The advantage is reducing the need for additional rotors; the four main rotors can provide motion in all three planes. The disadvantage will be slower yaw rates and increased maximum thrust requirements of each rotor basically allowing any two rotors to maintain required lift (a good requirement to have, not a great one to rely on everyday).
Putting the main rotors in tilt mounts and tilting them in rotationally-symmetric ways to induce yaw. It's an intuitive way to induce yaw, and the same tilt-rotor system would also reduce the amount of pitch required of the full aircraft to produce forward motion; just tilt the rotors and keep the fuselage level. Disadvantages include increased mechanical complexity, and the gyroscopic precession induced by trying to tilt a rotating mass which can cause the craft to tumble off its lift vector.

Other potential problems with the quad-rotor design include ensuring reliable power distribution (including maintaining some control if a rotor or an engine gives up completely), the lower inertia of the smaller rotors reducing the effectiveness of autorotation in an emergency, the need by many potential customers to fold the design up more compactly than its flight configuration, and the inherent fragility of the design with ducted fans on pivots in the corners of the aircraft, making it more vulnerable to fatal damage in "combat environments" (translated; everyone not on your side shooting whatever they have at you). Because the U.S. military has been a major customer of rotary-wing aircraft and a primary source of R&D funding, the survivability of the design against things like small-arms fire that a helicopter can typically take a few hits from is a definite point of consideration.
The last problem plaguing the quadrotor is more fundamental, and it is the lack of an answer to this simple question: what can a "full-size" quadrotor aircraft do that cannot already be done as effectively and cheaper with a traditional helicopter? The primary reason quadrotors make good UAVs is that it's easier for a computerized fly-by-wire system to keep them stable, but that is their only advantage to date compared to a single-rotor helicopter, so until the aircar becomes a serious legal and engineering possibility and people want a rotorcraft that is as easy to fly as a car is to drive (or at least easier than a helicopter where you pretty much cannot let go of the cyclic, ever), the quadrotor will likely remain small-scale.

Fantastic answer. Thank you. Just a little comment: Isn't bullet point 3) exactly how rotors are arranged in a quadcopter, by design?

Maybe not by design (meaning it's an absolutely essential design decision); the copter would fly stably with the rotors on each side rotating the same and opposite the other side (a dual twinrotor setup), but you would need an alternate method of yawing. That opposing-corner arrangement is a problem in itself for scale-up, because the amount of yaw produced even by all four rotors is much less than what you get with the single rotor of a chopper (for a number of reasons, primarily that the center of lift, center of mass and axis of rotation are all very close in a chopper)

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What is the purpose of the "Mirror-Thing" over the Mig-29 Cockpit?
I noticed that some Mig-29 Fighter Jets have a little mirror-like-thing over the cockpit. I can't imagine a possible usage from it (If it's really a mirror).
Here's a picture from Wikipedia:

As you can see on the following picture, not every Mig-29 has this thing:

What is this "Mirror"-like-thing supposed to do?
Why can't I find it on every Mig-29?
When is this thing used? In combat, training etc.?
How does it help the pilot / trainer in flight?

I think it's a periscope for the trainer

It is an actual mirror as you can see more clearly in this image:https://upload.wikimedia.org/wikipedia/commons/4/4f/Mig-29_on_landing.jpg.
I guess it is used to let the trainer have an unobstructed view.

Come on, @ratchetfreak, it is indeed a retractable periscope. Put this in an answer already and I upvote you. I hate to leave a question unanswered.

Asking more then one question  is not advisable but optimal.

Additional details to Marco's selected answer.
On the two-seater version, the instructor seat (aft) is not high enough, contrary to other training aircraft, to see the runway. A forward view mirror replaces the rear view mirror and allows the instructor to see ahead. The runway is projected on a second mirror inside the cockpit, creating a full periscope system:

Photo by Pavel Vanka. Modified.

Source

(Source)
Note how the projected part of the image fits very well into the direct view of the exterior.
An opening is created in the canopy when the top mirror is deployed. From below this is what it looks like (not exactly the same model):

Photo by Burkhard Domk. Modified.
The bottom mirror unfolds down to instructor eyes height:

(Source)

Very elegant and fairly cheap. Western trainer aircraft have favored "stadium seating" (trainer sits higher than pilot) coupled with a forward-looking CCTV camera, and an instructor familiar enough with the plane that he can land it blind.

Reminds me of the story about how NASA spent (m|b)illions of dollars to figure out the whole space pen, while Kosmonauts just used a pencil.

@WayneWerner -- the space-pen story is a myth.

@jjack https://space.stackexchange.com/a/1713/415 is relevant. One of the sources cited from https://en.wikipedia.org/wiki/Writing_in_space is http://www.physics.org/facts/apollo-nasa-pen.asp which also states that the USSR started using the same Space Pen at the end of the 1960s.

@WayneWerner - NASA originally used pencils as well. The pen was developed, yes, but not by NASA. It was a product of a pen company. NASA didn't spend anything to develop it but they did buy lots of the pen when it was demonstrated to them to work upside-down (thus should work in zero G)

It's a mirror for a periscope used by the instructor (sitting in the back seat) to see the runway on landing.
As reported here, two-seater versions of MiG-21 and MiG-23 also have the same feature.
In this picture, the periscope is, in fact, clearly visible on the back seat's canopy.

Image courtesy of Airvectors.net

The MiG-23's rear seat had really poor visibility, I think they even called it "The Coffin" or something, so the periscope here was a necessity for the instructor to see anything going on ahead


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Has a C-130 ever been designated as "Air Force One"?
Near the end of the movie, "Air Force One", Harrison Ford is transferred to a C-130, which then changes its call sign to "Air Force One". My question is, has a president of the US ever flown in a C-130 while in office?

Dunno about AF1, but my recollection is that a crew from Little Rock AFB, the 61st airlift squadron IIRC, did fly a VP somewhere, so in that case you had a C-130 flying with the Air Force 2 callsign.

I was Wing Commander of the 463rd TAW at Dyess AFB in 1992 when we flew President George H. W. Bush from Andrews AFB to Sanford, Maine, in one of our C-130s. We were Air Force One that day.

Wow! Great to have first-hand knowledge! Welcome to [aviation.se] and please browse the [tag:military] and [tag:us-air-force] tags, at least, for more questions you can authoritatively answer.

As I am sure you are aware, the convention is that any aircraft that has the current US president on board is given the "One" call sign.
For military aircraft, this is the branch of the military followed by "One", which is why you have Air Force One, Marine One, Navy One. If he is on a civilian jet, it is called Executive One.
The actual plane that everyone calls Air Force One is called VC-25, which is the military version of the 747.
There have been many planes that have been called Air Force One, from a King Air B90 to Gulfstream jets and even a combat aircraft like the S3 Viking (which was called Navy One).
However, as far as I know - no president has flown in a C-130 while in office.
Just to close this off, if a president is no longer sitting and is transported; then the callsign changes to SAM (Special Air Mission); so for the "Air Force One" 747, this would be SAM 28000 or SAM 29000 - depending on which of the two is being used.
The classified callsign for Air Force One is Angel.

So when someone says 'SAM incoming' you never know whether you should deploy flares and take evasive action, or whether it's just a surface-to-air missile. :)

That's not what they would say, they "Approach, this is Sierra Alpha Mike Two Eight Zero Zero Zero at 7000 for 3000 with Bravo" -- if you ever hear that or similar, look around ;)

Hrm... that call sign isn't so classified anymore, is it?

its also worth mentioning that when the President flies on his copter, it is designated Marine 1.  http://www.gizmag.com/sikorsky-marine-one-presidential-helicopter/31956/

Its not called Marine One because its a helicopter, its called Marine One because its the Marines and the passenger is the sitting President; so even if he was in an airplane but it was flown by the Marines, it would also be called Marine One.

And if he flies on something that can't decide whether it wants to be an airplane or a helicopter, it's probably also gonna be called Marine One :-D

Out of curiosity, what designation is used if the aircraft is neither military nor civilian, e.g. CIA or the like?

The idea of the designated call sign is to give priority for air traffic routing and other considerations (such as preparing the necessary security profiles and other restrictions). The choice of setting the designator is up to the mission commander (normally, the Captain). For clandestine missions - such as when George Bush (#43) went to visit troops in Iraq for Thanksgiving - he flew the VC-25, but the call sign and all flights plans for filed for a private jet; it was not until the plane landed that people realized it was "Air Force One" (ie, the President).

SAM is pronounced as one word on the radio, not spelled out phonetically.


text:
What is the aircraft loss-rate from Nimitz-class aircraft carriers?
What is the aircraft loss-rate from Nimitz-class carriers?   That is, what proportion of sorties result in the loss of the aircraft for reasons other than enemy action?
I was prompted to ask this after the Russian carrier Kuznetsov lost two aircraft in the space of a couple of weeks.

@mins While I agree that this is military related, we have many military related aviation questions, including many well received questions asked about aircraft carrier operations. I also don't see military questions listed as off-topic in the help center.

@mins What if it were a question about statistics for runway excursions for one type of runway or operation vs another?

@JonathanWalters: It would be interesting. The OP should clarify and emphasize the possible difference of landing techniques, or carrier designs.

I'm guessing the OP is asking directly related to launch and recovery, in transit the aircraft isn't any different from one that is land based. I'm thinking he is after recovery operations that result in a loss of airframe that wasn't because of damage sustained in combat. Basically pilot error in landing...

@RonBeyer You're reasoning makes sense, but your conclusion doesn't seem to follow. Aircraft can be lost in a number of ways without pilot error during landing. If he's asking about the number of aircraft lost on landing, then an edit would be in order.

Hi, thank you for all the above comments.  @Ron Beyer - I had a mind losses for all reasons other than enemy action, that is, accidents of all types.   Certainly I don't mean to restrict the question to pilot error only.   Reading reports of the Kuznetsov incidents, it appeared that both were related to problems with the arrestor gear.   If someone can indicate a breakdown of losses by cause or phase of operation, e.g. during takeoff, during landing, fuel exhaustion - that would be even better!

@Crosbie That may be difficult, as the US Military is not required (nor does it usually) release accident reports regarding its aircraft. The NTSB has a good database of commercial/private accident statistics and data, but I don't believe there is a good source of the information you are asking for. You'd probably have to rely on news reports. It isn't a bad question, but I don't believe there is a good way to get the breakdown/info you are looking for. Good luck though, hopefully somebody does have it.

The Naval Safety Center publishes some useful stats here.

(Rates are per 100,000 flight hours per year. Source.)

(Source.)
A few more details are available in the summaries. One example:

Other examples:

E-2C: Electrical arcing at vapor cycle power receptacle led to aircraft fire in the hangar bay.
A single F/A-18C crashed 6.6 miles northwest of Lakenheath, England. Event Cost \$72,000,000
MV-22: While recovering to LPD, MV-22 landed short of spot 2, main landing gear in deck edge netting
Engine fire leading to ejection. Event Cost \$71,000,000
Two CH-53 helicopters crashed while on training flight. All 12 crewmembers deceased.
AV-8B experience catastrophic engine failure during takeoff. Event Cost \$62,800,000
AV-8B experienced loss of thrust, pilot ejected safely. Event Cost \$62,800,000
2xF/A-18F Collided during Air-to-Air training. 2 aircraft lost. No fatalities. Event Cost \$173,580,662
EA-18G inflight engagement during night CV landing caused extensive damage to aircraft
F/A-18 impacted ground during air show practice. Event Cost: \$75,400,000
MH-60S crashed into James River while performing SAR training flight. Event cost: \$25,508,816
KC130T EXPERIENCED HAIL DAMAGE WHILE PARKED ON THE RAMP AT PETERSON FLD, CO
(KCOS)
F/A-18C crash during night air-to-ground training mission. Event cost: \$76,500,000
[snip]
T-45C: ACUTE PHYSIOLOGICAL EPISODE RESULTS IN ACTIVATION OF THE EJECTION SEQUENCE BY CREw. Event Cost \$37,800,000
A single T-45C impacted the ground at high energy. \$37,800,000
[snip]
Aircraft impacted a tree during low level training. Aircraft recovered safely.
[snip]
MV-22 ditched off Okinawa during NVD training mission. Event Cost \$80,600,000

Class A mishap definition:

\$2,000,000 or more and/or aircraft destroyed [or] Fatality or permanent total disability

Just note that these are just Class A's. It doesn't include B's, C's, or D's. Some of these aren't carrier related.
On a completely unrelated note, notice the costs. Modern a/c are expensive (although not all that is flyaway cost).

The math in the bottom table doesn't work... dividing the mishaps into the flight hours doesn't give the rates listed. Something else, not shown, seems to be in play.


text:
What would they call the US President's personal plane?
Air Force One is called that because it is operated by the Air Force.  The President's personal helicopter is called Marine One because it is operated by the Marines.
What would the President's personal aircraft be called?

Easily found on Wikipedia.

I'd expect Executive One:

Executive One is the call sign designated for any United States civil aircraft when the President of the United States is on board.

I actually heard it was Corporate One.

@wbeard52 never heard that one. Most likely it's some company's way to refer to their own aircraft when used by the president of the corporation, not the president of the country.

I don't remember where.  I think it was from a friend I was talking to.

@wbeard52 I thought you were making a political joke, which was a much better explanation. ;-)

Almost an example: Nelson Rockefeller continued to use his personal Gulfstream while Ford's Vice President and it was known as Executive Two.

Executive One is used while the President is on a civil aircraft according to FAA JO 7110.65W (.pdf). Apparently, this information hasn't been published in the AIM for a while.

(b)
  When  the  President  is  aboard  a  civil
  aircraft, state the words “Executive One.”

(Page 2-4-10)


text:
How do I enter the traffic pattern if I am coming from a nonstandard direction?
I have some questions on entering the pattern, VFR at an uncontrolled airport:
Scenario: let's say I am going to land on a one-runway airport (36-18). Also,
let's say I will be landing on 36 (left pattern) and I would like to enter left downwind.
I would like to know the proper pattern entry for left downwind for 36 for these different routes:

What is the proper entry if I am coming from 090°?
What is the proper entry if I am coming from 020°?
What is the proper entry if I am coming from 180°?
What is the proper entry if I am coming from 200°?

Where in the world are you located?

There's no such thing as a "non-standard direction". There's just a direction, and you should be taught to enter a traffic pattern from all of them. The question is fine, but the assumption that there's such a thing as a standard direction is misleading IMO.

Closely related, maybe a dupe?

Why not make all pattern entries 45 degrees to downwind?  Notice if you do something "different", someone could be coming not only from 45 degrees downwind, but also from a touch and go or aborted landing.  Assuming (better to check) the airport has a left hand pattern, one can maneuver their approach to the airport to "drop into" the pattern in a consistent manner, making it safer, and also beginning the process of setting up your landing.

Follow published procedures if available (of course).
Otherwise, there is no "proper" way other than that you stay out of the circuit area until you are at the right altitude and course to enter downwind (in Europe right angle to mid-runway, in U.S. at 45 degree angle).
E.g. if you come in from the opposite end of the entry, you can either fly over the circuit or make a large circle around the circuit area.


text:
Can a CFII log Dual Given if flying as SIC?
I am an MEI/CFII/ATP type rated and authorized jet pilot in Part 135 operation.  On the legs where I'm SIC slinging gear and working radios, is it ok to log this time as "dual given" and PIC time as an instructor?  Let's assume that I do in fact provide some form of instruction to the other pilot (whether or not this is wanted or needed).

Does the aircraft require a SIC? Something tells me by the sound of this that it might not be quite kosher.

Unless someone has a letter of clarification from the FAA legal counsel such as I might expect to return from this search - https://www.faa.gov/about/office_org/headquarters_offices/agc/practice_areas/regulations/interpretations/?year=all&q=Dual+given&bSubmit=Search , this is going to be primarily opinion based.

Related: https://aviation.stackexchange.com/questions/23810/can-a-non-instrument-rated-safety-pilot-log-pic-during-an-ifr-flight

It’s your logbook, so you can write in it anything you want. If a future prospective employer sees somebody doing what you described, the chances of being hired would fall drastically, since nobody in their right mind believes that the FO as PM is providing meaningful or valid “instruction” in a Part 135 or 121 environment to the Captain. If an FAA examiner sees that sort of time claimed as PIC to meet some requirement, I doubt the reception would be any better.
But, you go ahead & do what you want to do, since you will be the one who lives with the consequences. I think it’s a terrible plan, and I couldn’t see wanting to hire somebody who did that, not only because the PIC and Dual Given number are lower than shown, but also because of the brazen duplicity involved.
It’s playing with fire, and potentially risking a one-way ticket OUT of the airline industry. Which, for somebody who would actually do such a thing, would be the better answer.

Yeah, I wouldn't log instructor time for providing unwanted or unneeded advice to your captain...  It's just bad form.

Another way to look at this:  Ask yourself if the captain is a student pilot who requires a CFI.  If a CFI is not required then you can't really justify logging instructor time just because you are one, right?!

You're asking two questions here: can you log dual given, and can you log PIC. I assume you already know that you can write anything you want in a logbook; the FAA only cares about it when you use logged time to meet their experience requirements for something. Employers and insurance companies may have their own, different expectations, though.
Anyway, "dual given" isn't defined or required in any regulation so you can log whatever you want for that. Obviously if you later apply for a job that asks for a certain amount of dual given time, you'll have to be able to justify it. I'm not a CFI, but as far as I know most CFIs log dual given only when they also endorse a pilot's logbook to record the training (61.51(h)). Building up a lot of dual given time when there's no record of anyone actually receiving training seems like it would be an invitation for trouble if the FAA or an insurer ever decides to 'audit' you for some reason.
Next, logging PIC time. I guess you're thinking of logging the time per 61.51(e)(3):

A certificated flight instructor may log pilot in command flight time
  for all flight time while serving as the authorized instructor in an
  operation if the instructor is rated to act as pilot in command of
  that aircraft.

It isn't completely clear from what you've said above if you're rated to act as PIC, or just SIC. As far as I understand Part 135 - which isn't much - the operator might have their own OpSpec rules on who can act as PIC. But even if you are rated as PIC, would you also be acting as an "authorized instructor in an operation"? That seems very unlikely, especially considering your comment that you might log training given to the other pilot "whether or not this is wanted or needed". Unless you have specific authorization from the operator (your employer, I assume) to instruct, then I don't see how you could claim that you meet what the regulations require.
Finally, like others who have responded, I have the feeling that you want to push the boundaries a bit too far here. In particular, if the other pilot neither wants nor needs instruction, how would it even work? Would you just talk at him and then claim you gave him instruction? Instruction that isn't recorded in any logbook apart from your own? For me personally, it just doesn't seem to pass the "could I justify this to an FAA inspector?" test.


text:
Which frequencies does VDL Mode 2 use when used via the ATN (rather than ACARS)?
If I understand the slide below correctly (source), CPDLC can be used either via ACARS or via ATN.
I know that, in the case of VDL Mode 2, there are specific ACARS frequencies. Which frequencies are used when CPDLC runs over ATN?

Some additional information (and corrections)...
The slide is not totally correct (sorry for Airbus) as the same VDL-2 ground stations are used to transport both FANS-1/A CPDLC (A623) and ATN CPDLC over the AVLC protocol (VDL-M2). The two networks are interconnected... But depending on the media access scheme (Plain Old ACARS (POA - refered as VDL-A on the slide) or AOA (VDL2)) the modulation and the frequencies used differs.
In Europe, VDL-M2 is used between 136.7 and 137 (ICAO).
As by 2020 ATN-B1 CPDLC above FL285 will be mandatory, datalink will only be performed on this band using VDL-M2 and no POA.
However, it is true to say that there are two CPDLC. The  ICAO-standaridzed  one (or compliant to the ICAO-ATN  -- as used in Europe) and the non-ICAO compliant initially developed by Boing (as used in the US and oceanic -- also known as FANS-1/A). The two CPDLCs have very similar features and capabilities contrary to what have been told previously. The difference between the two is the end-to-end performance requirements and insurance of delivery (PM-CPDLC - PM=Protected Mode) available on ANT CPDLC and not for FANS-1/A CPDLC (there is no way to ensure a message have been successfully transmitted with FANS-1/A).
FANS-1/A CPDLC is transmitted via the ACARS network using Plain Old ACARS (POA - refered as VDL-A on the slide) or AOA (VDL2), while ATN CPDLC is only transmitted via VDL2 (SATCOM is being validated at the time of writing).
Link2000+ is the name of the development project of VDL2 (US and Europe).
ATN-B1 refers to the implementation of specific CPDLC services and/or messages standardized by ICAO. At the time of writing ATN-B1 is implemented in Europe and tests are being conducted for ATN-B2. ATN-B2 adds some new CPDLC messages but also the ADS-C which is the precursor for I-4D trajectory management.
ATN-Bx refers to the ground implementation, and FANS-xx refers to the airborne implementation (FANS-2/B refers to the ATN-B1 airborne implementation, FANS-3/C to the ATN-B2 -- Don't try to find logic here ;-) )
Have a look in the book "Aeronautical Air-Ground Data Link Communications" for further information between the two (with some historical background).


text:
What is the difference between centripetal and centrifugal force?
In a homework problem I was asked:  In a balanced banked turn an increase in angle of attack will :?
Answers are :
1/ reduce indused drag.
2/ increase centripetal force .
3/ increase centrifugal force .
4/ have no effect on the turn.
Obviously answer 1 and 4 are wrong but what about centripetal vs centrifugal force?  What is the difference between the two?

Have you made any effort to answer it yourself? What do you think about the options?

I think Its increase in centripetal force

Can you explain why that would be?

@AmranAlbalushi:  Centripetal force is correct but please don't just ask your homework questions on this site.  An appropriate question might be what is the difference between Centripetal and Centrifugal force.  A question worded this way will help others who have the same question.

@DLH the thing is I’m not quite sure about so I want to understand it well I might be wrong about so I want someone to clear it for me that’s why I’m asking this

So can someone explain me why ?!

@AmranAlbalushi:  At the risk of angering my colleagues  I will go and answer the question since you are new.  However questions asked on this site should not simply be homework questions asking for the correct answer.  If you are confused about a certain concept (like difference between centripetal and centrifugal force) just ask that.  If you don't mind I will edit your question so you don't get more down votes.

Please do  as I’m new here so I don’t know how this site goes

There's an xkcd for everything these days.

I enjoy drowning in terminological quagmire.

Centripetal force is the answer.  Hopefully you can see that options 1 and 4 are wrong without me having to explain.  I will elaborate instead on the difference between Centripetal force and Centrifugal force.
First Centrifugal force is not really a force at all but really a perceived force due to the inertia of an object to resist a turning tendency.  In a coordinated/normal 1G turn you should not feel any Centrifugal force.
Centripetal force is a force that makes a body follow a curved path.  When the angle of attack is increased more lift is generated and hence there is more force to execute the turn.  Since the wings are banked the increase in lift causes the airplane to turn more so Centripetal force would be the correct answer.
EDIT:  Due to some comments about my answer I decided to clarify a few things.  First of all where there is Centripetal force there is centrifugal force with centrifugal opposing Centripetal force.  However in an airplane there are many forces at play.  In a coordinated 1G turn, the horizontal lift component matches the centrifugal force (but in opposite direction) as shown in the diagram below.    Therefore the horizontal lift cancels out the centrifugal force so you don't feel the centrifugal force but it doesn't mean the centrifugal force isn't there.  Forces in an airplane can be deceiving that is why pilots sometimes get disoriented when there is no horizon for a reference.

@AmranAlbalushi, the second paragraph is totally wrong, unfortunately.

In a coordinated turn you definitely do feel the centrifugal force. The centrifugal force is indeed a perceived, better known as inertial, force, but the cause is acceleration of the reference frame, the one attached to the turning aircraft, not inertia of anything. Centrifugal force exists in any and every reference frame attached to a turning or rotating body. It causes acceleration exactly opposite to that caused by the centripetal force in the corresponding non-rotating frame of reference.

On a side-note, the Earth-attached frame of reference is not inertial either as it contains the inertial force of gravity. To complicate matters further, according to standard English terminology, gravity is not just the gravitational force, but the sum of inertial forces, which include gravitational forces (from Earth and other celestial bodies) and centrifugal forces due to rotation of Earch and all it's orbits).

If 2 is correct then 3 is also correct. If you increase one then you increase the other, period. They are linked.

The “feel” term the way you used it is bogus. The fact that two forces are balanced does not mean you don't “feel” them. Since the centripetal force (= horizontal component of lift) is acting on the wings, and then transferred to your body by the seat, while the centrifugal force is an inertial force, acting on each particle in the reference frame attached to the aircraft, they act at different places and therefore create stress in your body. You definitely do feel that. It is pushing you into the seat more than in straight flight.

Hello guys  as I went for exam AT CAA and this question came up I answered increase centripetal force it came wrong . So now the CAA want prove from me that my answer is right .

@AmranAlbalushi:  That is suprising to me that you got is wrong.  Check this link out: https://en.wikipedia.org/wiki/Banked_turn#Banked_turn_in_aeronautics it specifically mentions centripetal force.

I’ll check that and I’ll show them this

This answer could be improved by deleting the diagram, unless it mean to be an example of a wrong understanding, in which case that should be noted in the text.

I'm not the least bit surprised he got it marked wrong (2 comments above); the fact that diagrams like this have existed in pilot training manuals and exams prep materials and on actual exams for many decades shows that the people who write the exams have a false understanding of what is going on in flight.

Two of the given answers - choices (2) and (3) - are correct in different reference systems. It depends on the reference system of the observer:

In a non-accelerated reference system, centrifugal forces do not exist. Increasing angle of attack increases the centripetal force, that increases the turn rate. The reaction force of the centripetal force is the innate force of the plane.
A passenger in the plane observes the scenario from within an accelerated reference system. The plane is turning without obvious reason. The passenger's body wants to move in a straight line and seems to exert a force on the seat. This virtual force is called centrifugal. The seat seems to react by supporting the body with the same amount of force. When the pilot increases the angle of attack of the balanced turn, the centrifugal force will increase. The observer inside the rotating system is not aware of the centripetal force. The centripetal force does not exist for him.

Centripetal and centrifugal forces never co-exist in the same reference system! Even though they might appear like counter forces to each other, they should not be understood like that.
Centrifugal force is a construct to explain our perception and sometimes to simplify calculations in rotating reference systems.
Between the two, centripetal force is the more fundamental notion. It really describes what is going on, therefore (2) is the better answer from a scientific point of view. Nevertheless, (3) centrifugal force is a valid answer if the reference to a rotating system is established. That seems to be the case in the question. The centrifugal force describes what an occupant feels who is not aware of the rotation.

We may wish to consider both centripetal and centrifugal forces as archaic terms.  Acceleration, and the load created by acceleration is another way of seeing it.  David swung his sling with centripetal force on the string, but the stone flew in the instantaneous direction of travel when the string let go.  So it was released when the stone was moving at the target, roughly 90 degrees from the line between the slinger and the target. The actual angle takes into account the length of the string, forming a triangle between the slinger, target, and position of stone at release.

@RobertDiGiovanni "the load created by acceleration"* This is a very misleading way of seeing things. Accelerations don't cause forces (loads in your wording), forces cause accelerations.

Yet we call them G forces, which could be considered doubly wrong.  This forum has many times helped much to clarify what are essentially differences in language rather than concepts. Happy to be here.

This is a good answer-- I am going to offer a slightly different one along the lines of the comments above, feel free to comment on it.

Centripetal force is an interaction force and as such exists in all reference frames and is the same in all of them (at least under Newtonian mechanics; I am not completely sure about relativistic mechanics). They therefore do coexist in the rotating frame of reference (the one attached to the plane).

Dear @JanHudec, would you agree that Centripetal Force is defined as a force that acts perpendicular to the motion of a moving body?

@bogl, no, I would not. I would define it as a force that acts in direction towards a centre. Yes, in the rotating frame of reference the object is not moving, but it is the very same force that you call centripetal in the non-rotating frame, so calling it differently in the rotating one makes no sense.

@bogl, on on other words, you can argue that the force should not be called ‘centripetal’ in the rotating frame of reference, but the force itself definitely exists there. In contrast to (inertial) centrifugal force, which is actually effect of the reference frame rotation and therefore does really not exist in the non-rotating frame.

@JanHudec If you don't accept the definition of a fundamental term, it is no surprise that you have trouble accepting the rest.

@bogl, the force is called centripetal, because it keeps the body in circular motion, as viewed from inertial motion. Since it is a ‘real’ force, it exists in any and all reference frames. Suddenly stopping to call it ‘centripetal’ is a very different notion from the force not existing. Especially since the later is the case for centrifugal force. Well, inertial centrifugal force anyway, because some also define ‘reaction centrifugal force’ as the force exerted by the occupants on the aircraft and that also exists in both reference frames (but acts on different object).

… if you said that the force is not called centripetal in the rotating frame, I would have no problem with it. But the force exists in any reference frame. Different names don't change that.

@JanHudec You are right only as far as there are always two opposed forces. One is always a reaction to the other one. Which one is which is symptomatic for the choice of reference system. Global observer sees a centripetal force acting. A rotating observer (who is not aware that he rotates) believes to feel a centrifugal force acting. It's never both at the same time. And yes, this is all about names - that IS the point!

@bogl, no, there are always two opposed interaction forces, acting on the aircraft and air respectively (principle of action and reaction), but the centrifugal force in the rotating frame of reference is in addition to that. Inertial forces except gravitational force don't follow principle of action and reaction. Also, it is about the names, but once you give some force in the situation a name, it should keep that name even as you switch between reference frames, because it might not be the name you'd give it in the other frame, but it still the same force.

@JanHudec The issue, I think, is that most things you say are true in an inertial ref frame, which the rotating frame is not. The centrifugal force is merely a crutch to look at a rotating system as if it was not rotating. No rotation - no centripetal force. It's a notion, not part of any reality!

@bogl, of course it is a notion and notions are not part of reality, just human thoughts. But the point of that notion is to be able to look at situation from one point of view and know how it necessarily looks from another point of view. So when you say there is a centripetal force in the non-rotating (well, it is rotating too anyway) frame and you switch to the rotating one, there is a difference between saying the force is not called centripetal there and that it does not exist, because it still does. Remember, you are describing the same situation.

@JanHudec There is a pair of opposed forces that always exists. I never wrote anything else. Depending on how you look at it, it makes sense to call one of them Centripetal force, or not. Is that really so hard to understand?

What do you mean by "the non-rotating reference frame is rotating, too"?

@bogl, we are generally using the Earth reference frame as if it was not rotating, but Earth is rotating, orbiting the Sun and that in turn is orbiting the Sagittarius A, making it at least three rotating motions. We are just far enough from the centre of rotation that we can consider the centrifugal forces as uniform (zero divergence) and bundle them into gravity, also treated as uniform.

Let us continue this discussion in chat.

This is all wrong. There can be no balance of forces in circular motion, otherwise you would not have circular motion. For circular motion you need only centripetal force. Centrifugal and centripetal force do not coexist in the same reference frame so they can not be in balance or cancel each other out. In airplane reference frame there is only centrifugal force and gravity, which both add to resultant G load.

A "balance of forces" would seem to imply no net force, and thus no acceleration. But an object with circular motion is continuously changing its velocity vector -- thus, "accelerating", so it clearly is being acted upon by an unbalanced, net force.  So I entirely agree with you.  That said, this might be better as a comment than an answer, once you have a little more reputation.  Welcome to Av.SE!

Definitely bring this discussion to your teacher and classmates.  The Centripetal force is the vector of real forces that points toward the center of a turn.  No better example exists than a object in orbit.  Orbital motion is a curve produced by gravitational acceleration downwards and constant velocity to the side.  There is no centrifugal force.  "Centrifugal force" as the name implies, is the G load created by acceleration in the opposite direction. Note, at constant velocity, in orbit, the ACCELERATION towards earth cancels out gravity, resulting in "weightlessness".

@RobertDiGiovanni  Only in physics class all masses tend to be either points  or solid regular spheres. Reality is more complicated. Our oceans are bulging out towards the sun, and away from the sun as well. Centrifugal force is an elegant way to explain the latter.

Santa Claus is "elegant" too.  Frame of reference is more like it, or action/reaction.  And let's not forget the motion of the solar system around the galactic center.  Yes, I know reality is complicated, that is why applications interests me quite a bit more.  In all cases "centrifugal force" is the RESULT of a change in direction.  However, we could discuss why we call them "G" forces, when gravity is an attraction of masses.  Maybe because they are combined with gravity to produce net load.  I'll go along with "C" forces in that context.

@RobertDiGiovanni G-forces are called so because they are given in (pseudo) units of the gravitational constant (g). People like calling physical quantities by their unit, rather than by their intriguing physical names. And the difference between reference systems and Santa is that he never helps you calculating a problem.

The whole "g force" confusion is mostly pilotspeak, in my experience; in more analytical environments we use load factors, which are of course non-dimensional. Precisely to avoid all this talk of "Gs".

As answers get shifted around, it becomes unclear what answer your opening sentence refers to, or whether it refers to the original question.  We aren't really supposed to focus on other answers anyway in StackExchange but my point here is just make clear what your sentence is referring to.

In the reference frame of the aircraft, the centripetal and centrifugal forces do coexist, and cancel each other out, which is why the aircraft remains in place in that reference frame. Otherwise the plane would have to accelerate along the centrifugal force.

2. and 3. are both true, because centripetal and centrifugal force are intimately related by the principle of action and reaction (a.k.a. Newton's third law of motion).
However if you are expected to only check one answer, the examiner has probably drowned in the terminological quagmire surrounding these two terms, in which case they probably want to hear 2.

What is the difference between the two?

Centripetal force is a force that is causing a circular motion. In case of turning aircraft, it is the horizontal component of lift.
Centrifugal force, in contrast, is an inertial force in the reference frame of the turning object that balances the centripetal force there so the turning object—the aircraft—stays in its place in that reference frame. In that reference frame, the centripetal and centrifugal force are action and reaction and as such have the same magnitude and opposite direction, always.
In Newtonian, classical, mechanics, inertial forces are called “fictitious”, because the laws of motion are postulated for inertial reference frames and these forces are considered just artifact to allow using them in non-inertial reference frames as well. In this context, answer 2. makes sense alone more than answer 3. alone.
However in General relativity, the inertial forces, which in General relativity includes gravitational force¹, are usually considered just as real and all reference frames just as good as the ones in free fall (which take over the role of inertial ones), so centrifugal force is just as real and both answers should be checked.

¹ I intentionally didn't write “gravity”, because in usual English terminology that is used for the sum of inertial forces in the reference frame of Earth surface, which includes gravitational force of Earth, centrifugal force due to rotation of Earth and tidal forces due to gravitational forces of other celestial bodies and orbital motions.

With due respect, but centripetal force is not the reaction to centrifugal force. Please see my answer.

@bogl, you say there that in the rotating reference frame the centripetal force does not exist. But it being “real”, aerodynamic, force, it is invariant to coordinate transformation, so it must exist there—it is what keeps the aircraft from being accelerated by the centrifugal force, which makes it a reaction to the centrifugal force. So how do you call it in that reference frame instead?

The rotating frame is really useful for rotating rigid bodies. Inside the plane, everything makes sense. The pilot exerts centrifugal force on seat, seat reacts with opposite force. When turning, there is no solid string between the plane and the center of rotation. The surrounding air is neither rotating with the plane, nor it is solid. There is the horizontal component of the lift that happens to cancel the centrifugal force on the plane. The point is, that the rotating reference is not useful to  justify why this is the case. The forces match because of how the reference frame was chosen.

@bogl, The forces match because of how the reference frame was chosen, but they still match, they are still action-reaction pair, and they are still centrifugal and centripetal force. It is not particularly useful for understanding how aircraft turn, but it is still the definition of the terms—and since the centrifugal force only exists in the rotating reference frame, it is the only one in which it can be described.

By definition, a centripetal force makes a body to follow a curved path. There is nothing rotating inside the rotating reference. The notion of a centripetal force does not belong there. It is either centrifugal force against horizontal lift in the rotating reference, or centripetal force against innate force in the global reference.

Both, centripetal force and centrifugal force are acting forces. Neither of them can be regarded as a reacting force.

@bogl, centrifugal force is acting force in the rotating frame of refernce, centripetal force is acting force in the inertial frame of reference, but it is a reacting force in the rotating one. And not that it was strictly defined in all situations anyway.

As you wish. I give up.

There are really two different questions here.  You asked "What is the difference between centripetal and centrifugal force", but there's also an implication that you'd like to know what is the best answer to the homework problem.  This answer will try to address both questions.
We could say that in a constant-speed coordinated turn, the real aerodynamic force is equal to the wing's lift vector, which if we desire we can break down into centripetal and vertical components.  There is also a downward force component due to gravity.  The net force components including gravity are in balance in the vertical dimension, but NOT in the horizontal dimension-- otherwise there would be no turn.
The forces the pilot "feels" are only the real aerodynamic forces, not gravity -- or perhaps it is more descriptive to say that the pilot "feels" an "apparent force" that is equal in magnitude and opposite in direction to the net aerodynamic force.  So as the net aerodynamic force is transferred through the aircraft structure to the pilot's seat to the pilot's body, the pilot "feels" an "apparent force" pulling him down into his seat, which if we desire we can break down into centrifugal and downward components.  The root cause of this "felt" or "apparent" force is the acceleration acting on the pilot's body-- excluding the component of acceleration due to gravity.  Note that gravity does not actually "cause" the downward component of this apparent force -- if gravity were to instantly disappear the trajectory of the plane and pilot would both instantly change as the flight path curved upward into a loop but the pilot would not feel any change in the "apparent force" pushing him into seat.  The fundamental reason for this is that gravity "works from within" and exerts an equal acceleration on every molecule of the pilot's body and aircraft without causing any stresses or strains (ignoring tidal effects), so it isn't perceived as trying to squish the pilot's body down into the seat.  (We'd have to modify this point of view if we wanted to adopt a reference frame centered on the aircraft, treating the aircraft as a stationary object or an object moving at a constant velocity-- this would not be a valid inertial reference frame and would not tell us anything about the forces that a pilot actually "feels" in flight.)
In a constant-speed coordinated turn, thrust and drag are equal, and the plane is not being allowed to fly sideways through the air, so the airflow is not striking the side of the fuselage and generating an aerodynamic sideforce.  So the net aerodynamic force is simply equal to the wing's lift vector.  This force is acting straight "upward" in the aircraft's reference frame-- i.e. in same plane as the vertical fin-- and in response, the "apparent force" that is "felt" by the pilot is pulling him straight "down" into his seat-- i.e. in the same plane as the vertical fin.
Now what happens when we increase the angle-of-attack, causing at least a temporary increase in lift?
Let's take the point of view of looking at the real forces, not the "apparent forces".  If we increase the wing's angle-of-attack, we increase the lift force, including the vertical and horizontal (centripetal) components.  But we don't change the direction of the lift force.  Saying that we've caused a change in centripetal force is true, but incomplete, because we're not mentioning the change in vertical force.
Now let's take the point of view of looking at the "apparent forces", not the real forces.  If we increase the wing's angle-of-attack, we increase the lift force, including the vertical and horizontal (centripetal) components. This means that there's an increase in the "apparent force" pulling the pilot down into his seat, including both the vertical and horizontal (centrifugal) components.  Saying that we've caused a change in the apparent centrifugal force is true, but incomplete, because we're not mentioning the change in the apparent vertical force.
If we just note the increase in the "apparent" centrifugal force, we might think that when we increase the angle-of-attack and lift force, the slip-skid ball (and the pilot's body) will tend to deflect (lean) toward the outside wall of the cockpit.  This is not the case -- even though some of the very faulty diagrams we see in pilot training manuals and FAA exam materials might lead us to think otherwise.
The choice of 2) or 3) to the original question depends on whether we are interested in real forces or "apparent forces".  But neither is a complete answer because both ignore the vertical force components at play.  (If the question were about an increase in bank angle rather than an increase in angle-of-attack, then it would be a different story.)
2) is really a better answer than 3) because the question just asks about forces, not "apparent forces".
In the specific context of a coordinated turn, we could say that "centripetal" force is one component of the actual net force at play, while "centrifugal" force is one of component of the perceived force or "apparent force" at play, which is equal and opposite to the real force.
But more generally, "centripetal" means acting toward the center of the curve defined by the curving flight path (horizontal turn, or loop, or whatever), while "centrifugal" means acting away from the center of the curve traced out by the curving flight path (horizontal turn, or loop, or whatever).
We could have a discussion about performing loops that would be very similar to the discussion above.  Again, the actual net force would have a "centripetal" component-- as would the portion of the actual net force that is due to aerodynamic force-- while the "perceived" force or "apparent force" would have a "centrifugal" component.
Yet there are other cases where we can generate an actual aerodynamic centrifugal force component that reduces the total aerodynamic centripetal force that the aircraft is generating.  So it's not as simple saying that "centripetal" always refers to real force and "centrifugal" always refers to "apparent force".  Example-- starting with a coordinated turn-- now apply lots of outside (top-side) rudder-- nose yaws up/out, airflow strikes side of fuselage creating an aerodynamic force toward high wingtip-- this is a real force, and it has a centrifugal component, so net centripetal force is reduced and the turn rate slows.  Note also that when we add this new aerodynamic force into the picture, the total aerodynamic force is no longer acting in the same plane as the wing's lift vector, i.e. no longer is aligned with the vertical fin.  Therefore the slip-skid ball will ride off-center toward the low side of the cockpit, and the pilot's body will tend to lean in that direction as well.
And to add yet another twist, consider an aircraft doing multiple loops without stopping.  What is happening at the bottom of each loop?  Gravity is a contributing a real centrifugal force component that affects the rate and radius of curvature of the flight path, yet the only "apparent force" the pilot "feels" is the apparent "centrifugal force" component that is exactly equal and opposite to the aerodynamic centripetal force generated by the wing.
Likewise we can think of a situation where the NET aerodynamic force is centrifugal rather than centripetal in nature.  Example-- aircraft is flying an arcing trajectory such as the well known "Vomit Comet" zero-gravity simulator.  We'll focus on the instant at the top of the curving arc, where "centripetal" is the same as "earthward" and "centrifugal" is the same as "skyward".  Net aerodynamic force is zero, net "apparent force" is zero, total net force including gravity is equal to the weight of the aircraft and contents acting in the downward (centripetal) direction, and net acceleration is 1-G downward.  Now if we repeat the same maneuver but with the wing generating a very small amount of lift-- say 1/10 the total weight of aircraft and contents-- we'll get almost the same arc.  The net aerodynamic force is 1/10 the weight of the aircraft and contents, acting in the skyward (centrifugal) direction, so the "apparent force" acting on the aircraft, or on any object in the aircraft, will be equal to 1/10 the weight of that object, acting in the earthward (centripetal) direction.  In other words, we'll "feel" 1/10 "G" of acceleration toward the earth.  Our G-meter will read positive 1/10 "G".  But the total net force including gravity is 9/10 the the weight of the aircraft and contents, acting in the earthward (centripetal) direction.

The problem with the faulty diagrams we see in flight training manuals and FAA study/exam materials is they give the impression that the reason a turn feels "coordinated" is that centripetal force and centrifugal force are in balance, and that the reason a turn feels "uncoordinated" is that centripetal force and centrifugal force are not in balance.  I hope my answer helps clarify why this is a completely inaccurate way to look at things.  Some of the other answers did as well.

Some of the examples in this answer such as the aircraft at the bottom of a loop during a whole series of loops made the simplifying assumption that thrust and drag were equal, when that is surely not true-- hopefully that doesn't obscure the basic point being made here.


text:
How do I calculate the aircraft's deviation from track in this exam question?
I’ll have my PPL exam soon. I asked my instructor at the school about this question and even he doesn’t know the answer. I’ll appreciate your help.

Question 22
Flight from point A to point B. NDB is point B.

magnetic track (MKK)=245°
magnetic variation (“delta” m)=5° W
magnetic compass swing ( “delta” k)=3° E

When 30 km inbound to point B, your QDM-225° What is aircraft
  deviation from track?

Right of track 10km(5.4 nm) (Nk=+10km)
Left of track 10 km (5.4 nm) (Nk=-10km)
Right of track 6km(3.2 nm) (Nk=+6km)
Right of track 12,55km(6.7nm) (Nk=-12.5km)

You might like to add what you've already tried and what you don't understand. People generally respond better to questions like this if they see you've already made some effort, and understanding your difficulty helps to get a better answer.

As I understand my actual track is MT =245” and QDM  is magnatic track from the station as I’m going to point B what it is QDM =225”  and I have to get the track error what it is 5,4 NM  . I just don’t know how I know if I’m left of Track or right of track ?

QDM is magnetic bearing from aircraft TO station

Pretty easy... You are 225° QDM, the track was 245°. Neither QDM nor MKK is nothing about variation or deviation, because both of them are magnetic (i. e. variation already applied) courses (i. e. not heading, therefore deviation is also not an issue).
20° deviation from track at 30 km to the station gives us 30 * sin(20°) = 10.26 km, i. e. approximately 10 km. Our QDM is less than desired track, and we are moving to the South-West, so we are to the North-West of our desired track, i. e. we are on the right side of the desired track. Therefore, the answer is 1, right of track 10km (5.4 nm) (Nk=+10km).

I want to know now , that which one is my actual track is it the MT or the QDM ? Dose the question means that I was on MT but suddenly my A/C drifted to QDM or what ? That what I don’t know  which one is my real track ?

Your actual track is neither MT not QDM in that case, because actual track can be even curved (nobody knows how you deviated from your intended track: flew different course all the time until obtaining bearing, or suddenly turned perpendicular to your intended track, or failed to maintain one direction and wandered all the time, or whatever). And QDM is not track at all, it is a bearing, i. e. magnetic direction to the station.

ok got you thank you so much


text:
What is the definition of "occupant" in 14 CFR 91.307?
Does a solo pilot performing aerobatics constitute an "occupant" for the purposes of requiring a parachute per 91.307(c)?

Are they occupying the aircraft?

If you're solo you don't need a parachute (emphasis mine):

Unless each occupant of the aircraft is wearing an approved parachute,
  no pilot of a civil aircraft carrying any person (other than a
  crewmember) may execute any intentional maneuver that exceeds [...]

Incidentally, 91.307 says nothing about aerobatics; that's in 91.303.
And this question is closely related.


text:
Why are there many pilots who don't say "mayday, mayday, mayday" to declare an emergency?
I've been listening to some ATC from some major commercial pilots and in every video where there's been some confusion about declaring an emergency they've used some terminology other than "mayday, mayday, mayday" (here's an example).
Given that how unmistakable saying the word "mayday" is three times in succession to people of different languages and accents, and that it's much quicker than saying "declaring an emergency, declaring an emergency, declaring an emergency", I'm trying to understand why the "mayday" terminology isn't universal. I can't come up with any situation where a triple mayday call won't be the clearest and quickest way to indicate an emergency situation, regardless of whether or not the controllers and pilots speak English or not?
Are there any good reason for not using triple mayday for declaring an emergency? Are there some ATC controllers that wouldn't know what it means? I just don't understand because it seems to be clearly understood every time whereas alternative often aren't, even with two native English speakers.

Per AIM 6-3-1(d), "the word MAYDAY commands radio silence on the frequency in use." If that's what I need, I'll say it.

Did you consider that it might be nothing more than a matter of practice?  How many pilots ever have more than one occasion to declare an emergency in an entire career?  Personally, while I've never had to declare an emergency, I think I'd be too busy flying to concentrate on using proper communication terminology :-)

As a complete non-pilot, mayday would be the first word that would come to my mind a fast paced and panicked situation, just because it's so prevalent in popular culture. The "declaring an emergency" terminology is only something I learnt after listening to ATC videos.

In my very, very limited experience, what happens is that the aircraft is already talking to ATC and an 'issue' escalates to an urgency or emergency situation.  Since there's already a conversation going on and the flight crew are concentrating on the 'aviate' and 'navigate' bits, actually saying the words 'mayday' or 'pan-pan' is a redundant distraction.  I could be wrong, this is based on three or four situations I've been privy to.

@DaveGremlin in the videos I've watched they almost always have in the communication "declaring an emergency" anyway. I'm not suggesting that "mayday" be added, I'm suggesting it replaces "declaring an emergency". Even saying "mayday, mayday" (twice instead of the standard three times) is far clearer and also quicker to say, as far as I can tell. Saying "mayday" once is very quick and I think still clearer than saying "declaring an emergency".

@Clinton - The one incident I had, I was talking to ATC about an airframe issue and discussing my return to the field and my joining preferences.  ATC asked me if I wanted to declare an emergency before I had thought about it - I was working the problem and concerned with getting down safe rather than exactly what terminology I used.  When I've heard others in trouble, it's been ATC that have brought up the urgency/emergency thing because the flight crew have had their hands full with flying the aircraft.

I'm glad everything worked out well, but I'm not understanding your answer. I'm asking why terminology "mayday" isn't used instead of "declaring an emergency", in particular when actually declaring, not just chat about declaring.

Let us continue this discussion in chat.

Doesn't Mayday come from French: m'aider?

@WalterMitty oui, c'est vrai.

I'm a controller, not a pilot, so I can only speak from my own perspective.
What we are taught in ATC school is that many pilots are reluctant to use the word mayday because they feel it might escalate a situation unnecessarily and potentially create a lot of paperwork. I guess, mentally, it seems like calling mayday is a significant, irreversible step which can be daunting to take. They might be afraid, assuming the situation goes well, that they have to explain why they decided to escalate the situation, get emergency services involved etc.
That's not really a resonable fear, though. In fact, as a controller, I would much prefer a pilot to call mayday as opposed to trying to make the situation seem less critical than it is. In an emergency, clear communication is absolutely essential. I've had several situations where pilots have had "minor technical issues" or an engine that is "a bit jumpy" or something like that, and it puts me in a really uncomfortable situation because I can't really tell how serious it is. And if I'm in doubt, I will just call the emergency services anyway. On the other hand, the one time I actually had someone call mayday, within 30 seconds, the firetrucks were on the move, all the runways were cleared and the aircraft in question was cleared to land. It's much easier to help you if you are clear about what you need!

Pilots believing themselves to be facing an emergency situation should declare an emergency as soon as possible and cancel it later if the situation allows.
The correct method of communicating this information to ATC is by using the prefix “MAYDAY, MAYDAY, MAYDAY” or “PAN PAN, PAN PAN, PAN PAN” as appropriate. This procedure, which is an international standard, is the single most effective means of alerting the controller to the need to give priority to the message that will follow.

Emergency Communications on SKYbrary
In the end, it is always up to the pilot(s) to decide what is the right thing to do. Every emergency situation is unique, and there isn't (and can't be) rules describing what exactly must be done in every single situation.

Is there any additional paperwork involved for either pilot or controller when an emergency is declared? I do think it's human nature to not want to make a mountain out of a molehill especially if they have to explain it later.

@TomMcW Probably there is. Else ruthless pilots could declare an emergency just to get a priority landing to be home for dinner and then just say "oops, sorry". I definitely doubt this.

@TomMcW There is always paperwork involved after any kind of incident, but nothing major. At least from the ATC perspective, the paperwork is exactly the same whether a mayday is called or not.

@TomMcW - Insurance companies often ask about 'incidents' and I think that may act as a disincentive to call a pan-pan or mayday

There was a great wings seminar many years ago about controllers declaring emergencies when pilots are unwilling or too busy. I wish I could find the audio they played of a pilot that just replied "unable" to a couple of simple instructions (in icing conditions) followed by the controller calling 5 or 6 other airplanes in quick succession cancelling descent clearances or turning them out of the way. It was really moving.

@TomMcW, well, yes, but it is the same whether you say “Mayday, mayday, mayday”, or you say “declaring emergency”, or you answer yes when the controller asks “and, um, are you declaring emergency?”, or even if the controller simply starts treating you like an emergency because you can't follow normal procedures.

"Mayday, Mayday, Mayday" tells ATC (and everyone else on frequency) that you have an emergency, but it gives them exactly zero information about the nature of the emergency or what you need other than getting their attention and time on frequency. If you're in a portion of flight where you don't already have the attention of the controller, then adding "Mayday" or "Pan-Pan" calls to the beginning of your transmission helps to get you that immediate attention. However, if you already have their attention and other people aren't trying to talk on frequency, then it's not as necessary.
Furthermore, when quick communication is necessary and/or the pilot needs to be focusing on flying and navigating the aircraft rather than communicating (as is often the case in an emergency situation,) adding the "Mayday" calls to the beginning of your transmission even when you already have the controller's attention may go from merely unnecessary to actively unhelpful.
Of course, when you're in areas where English isn't the native language (or if English isn't your native language,) then sticking to the standard phraseology becomes more important, both for the sake of ATC and the other traffic on frequency. Even when all parties involved are fluent, significant differences in accents can make communication less clear, so using clearly-recognizable standard phraseology is more helpful for clear communication.
As far as the situation in the linked video, I would say that was more a case of ego than unclear communications. There's nothing particularly unclear about,

"We can't land on 22, we're breaking off the approach, and if you don't give us runway 31R, we're going to declare an emergency."

The situation wouldn't really have been made any more clear by adding "Pan-Pan, Pan-Pan, Pan-Pan," or "Mayday, Mayday, Mayday" to the beginning of it and everything they did say would still need to have been said. Once American 2 became aware of the crosswind problem on 22L, they quickly needed to change to 31R and they needed to communicate that as quickly as possible to Tower. Adding something to the beginning of the transmission would have just made it take longer to communicate and at a time when pilot workload would have been quite high preparing to change runways near the airport. The pilots needed to be spending their time talking to each other and focusing on flying and navigating the airplane, not on talking to Tower.

Saying "If ... we're going to declare an emergency" is actually not declaring an emergency. That's just a conditional promise. The ATC made that fact clear too. Only after the pilot continued with "we are declaring an emergency" it was clear. So adding 3x MAYDAY in the beginning would have made a big difference.

@Curd True, the pilot could have just gone ahead and declared an emergency in the first call. They decided to try less escalation first so that it wasn't just totally out of the blue and the controller had a chance to work with them without having to escalate to an emergency. When that didn't work, then they immediately declared an emergency. Regardless, I would still say that their statements were 100% clear and unambiguous the entire time, while proving the information that ATC needed and trying to maintain brevity so that they could focus on flying the airplane during a high workload.

@reirab: That sounds like the intention was to say "This plane is going to land on runway 31R.  Expect to apply whatever level of emergency handling will be needed to make runway 31R ready to accept it."

If there's a way to communicate something using standard phraseology, then communicating using non-standard phraseology is definitely less clear. As the recording illustrates plainly the controller didn't actually understand whether it was an emergency, Also what if the "don't" in your quote had been clipped and the controller understood it to mean "don't vector us for 31R"? Definitely clearer to say "UNABLE. PAN PAN PAN American 2. REQUIRE 31R."

The first time I heard the recording, I thought after the approach the pilot withdrew the emergency declaration, but he actually reissued it. That's about as unclear as you can get.

Mayday is a protocol for "breaking in unannounced" you might say, or for broadcasting in the clear on an open frequency to whoever might hear you.  So if you are flying around VFR and need to broadcast that your engine just quit, or are in an airliner on the North Atlantic Tracks out of ground based VHF and radar coverage, you use Mayday for sure.
When you are already in routine communication with ATC with a transponder code assigned and are an identified target on someone's radar display, it's not really necessary.  You can say Mayday, but ATC just needs to hear the word "emergency", and even if you don't do that, setting the 7700 transponder code is sufficient.

That's simply not true. Mayday is certainly supposed to be used when talking to ATC. And 7600 is a code for com fail, not emergency.

@J.Hougaard I learned this mnemonic: "75 - man with a knife, 76 - I need my radio fixed, 77 - I'm falling from heaven".

@J.Hougaard, do you provided different services depending on whether or not someone uses the word mayday?

@MichaelHall Not necessarily. But it is easier to choose the right service to provide when pilots are clear about what they need

So if someone you have already been speaking with who has told you they are minimum fuel escalates their situation by declaring "emergency fuel" would you treat them any differently than if they keyed up and said "mayday" three times?  Because I am with John K, that it is primarily a tool for breaking into a crowded frequency, establishing the situation on a poor HF connection, and getting other non-emergency aircraft to shut up so that you have the controller's full attention.  In the course of conversational transmissions is isn't often necessary.

https://en.wikipedia.org/wiki/Avianca_Flight_52

No obviously I would not treat someone saying "emergency fuel" differently than someone saying "mayday". But the former is not correct phraseology, the latter is.

Yeah finger trouble... I touch type.

@J.Hougaard, I am not disputing that “mayday” is an appropriate and universally recognized phrase for initiating communication of emergency status, I am simply pointing out, (as many others are in comments above…) that if you have already established a clear conversation with ATC about your status and wish to escalate it to an emergency, that uttering these codewords don’t really add any value.  Also, I would dispute your comment that the words “fuel emergency” or “emergency fuel” are incorrect phraseology.  Unless you can back this with a reference...

By the way, your answer above is a very good one!

@MichaelHall I understand that that is your opinion, and in a given situation, it's not necessarily wrong.. However, it isn't what the rule book says. For example, see page 19-20 in this guide: https://www.skybrary.aero/bookshelf/books/115.pdf

Yes, but a "guide" (advisory) is different than a "rule book" (regulatory)  I am very familiar with written guidance on this matter, but the important thing is to understand intent.  There are times when it is absolutely appropriate, I am simply saying that if your situation has been clearly communicated and you are getting the handling needed it is extraneous and unnecessary to interrupt the conversation to say 3 magic words.  And it is not "required".  I tagged you in a sidebar discussion linked above because @Dave Gremlin says it better than I can.

P.S. I understand that you are not bound by the FAA, but this reference discusses use of the term "fuel emergency" and says nothing about any need or desire to preface it with "mayday":
https://www.faa.gov/air_traffic/publications/media/atb2012-1.pdf  My point is that "fuel emergency" or "emergency fuel" is correct and appropriate phraseology, whether they are used in conjunction with "mayday" or not. And your reference does not contradict this.

@PerlDuck The one I heard was, "75, taken alive; 76, caught a glitch; 77, going to heaven."